Wow, these neighbors really have some nerve...
I got an email last week about why camera phones come in handy. Sounds like this is another one of those times. It'd be pretty hard for them to deny something that got caught on camera.
Even then though, just because they wouldn't be able to deny it any more, doesn't mean they'd stop doing it.
That fence is sounding like good idea. Dunno if they exist or not, but what would be even better (and funnier) is if they made motion-activated sprinklers!!!
Oh -
look - they do!